Pseudo-melanism, also called abundism, is another variant of pigmentation, characterized by dark spots or enlarged stripes, which cover a large part of the body of the animal, making it appear melanistic. A deficiency in or total absence of melanin pigments is called amelanism.
The morbid deposition of black matter, often of a malignant character causing pigmented tumors, is called melanosis. For a description of melanin-related disorders, see melanin and ocular melanosis.
Melanism related to the process of adaptation is called adaptive. Most commonly, dark individuals become fitter to survive and reproduce in their environment as they are better camouflaged. This makes some species less conspicuous to predators, while others, such as black panthers, use it as a foraging advantage during night hunting. Typically, adaptive melanism is heritable: A dominant gene, which is entirely or nearly entirely expressed in the phenotype, is responsible for the excessive amount of melanin.
Adaptive melanism has been shown to occur in a variety of animals, including mammals such as squirrels, many felines and canids, and coral snakes. Adaptive melanism can lead to the creation of morphs, the most notable example being the peppered moth, whose evolutionary history in the United Kingdom is offered as a classic instructional tool for teaching the principles of natural selection.
Industrial melanism is an evolutionary effect in insects such as the peppered moth, Biston betularia in areas subject to industrial pollution. Darker pigmented individuals are favored by natural selection, apparently because they are better camouflaged against polluted backgrounds. When pollution was later reduced, lighter forms regained the advantage and melanism became less frequent. Other explanations have been proposed, such as that the melanin pigment enhances function of immune defences, or a thermal advantage from the darker coloration.
Melanistic coat coloration occurs as a common polymorphism in 11 of 37 felid species and reaches high population frequency in some cases but never achieves complete fixation. The black panther, a melanic form of leopard, is common in the equatorial rainforest of Malaya and the tropical rainforest on the slopes of some African mountains, such as Mount Kenya. The serval also has melanic forms in certain areas of East Africa. In the jaguarundi, coloration varies from dark brown and gray to light reddish. Melanic forms of jaguar are common in certain parts of South America. In 1938 and 1940, two melanistic bobcats were trapped alive in sub-tropical Florida.
In 2003, the dominant mode of inheritance of melanism in jaguars was confirmed by performing phenotype-transmission analysis in a 116-individual captive pedigree. Melanistic animals were found to carry at least one copy of a mutant MC1R sequence allele, bearing a 15-base pair inframe deletion. Ten unrelated melanistic jaguars were either homozygous or heterozygous for this allele. A 24-base pair deletion causes the incompletely dominant allele for melanism in the jaguarundi. Sequencing of the agouti signalling peptide in the agouti gene coding region revealed a 2-base pair deletion in black domestic cats. These variants were absent in melanistic individuals of Geoffroy’s cat, oncilla, pampas cat and Asian golden cat, suggesting that melanism arose independently at least four times in the cat family.
Melanism in leopards is inherited as a Mendelian, monogenic recessive trait relative to the spotted form. Pairings of black animals have a significantly smaller litter size than other possible pairings. Between January 1996 and March 2009, leopards were photographed at sixteen sites in the Malay Peninsula in a sampling effort of more than 1000 trap nights. Of 445 photographs of melanistic leopards taken, 410 came from study sites south of the Kra Isthmus, where the non-melanistic morph was never photographed. These data suggest the near fixation of the dark allele in the region. The expected time to fixation of this recessive allele due to genetic drift alone ranged from about 1,100 years to about 100,000 years.
Melanism in leopards has been hypothesized to be causally associated with a selective advantage for ambush. Other theories are that genes for melanism in felines may provide resistance to viral infections, or a high-altitude adaptation, since black fur absorbs more heat.
Melanism, meaning a mutation that results in completely dark skin, does not exist in humans. Melanin is the primary determinant of the degree of skin pigmentation and protects the body from harmful ultraviolet radiation. The same ultraviolet radiation is essential for the synthesis of vitamin D in skin, so lighter colored skin - less melanin - is an adaptation related to the prehistoric movement of humans away from equatorial regions, as there is less exposure to sunlight at higher latitudes. People from parts of Africa, South Asia, Southeast Asia, and Australia have very dark skin, but this is not melanism.
The term melanism has been used on Usenet, internet forums and blogs to mean an African-American social movement holding that dark-skinned humans are the original people from which those of other skin colour originate. The term melanism has been used in this context as early as the mid-1990s and was promoted by some Afrocentrists, such as Frances Cress Welsing.
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